The present invention related to an automatic transmission for vehicles, and, more particularly, to a control system for changing over a transmission between a high speed stage and a low speed stage according to the balance of a throttle opening of the engine and vehicle speed.
A transmission for vehicles which has actuators and is adapted to be changed over between a high speed stage and a low speed stage according to operation of the actuators is known and is used as an overdrive means, an auxiliary transmission or a two-stage transmission for vehicles. A transmission of this kind is generally controlled by an electronic control system including a throttle sensing means which measures the throttle opening of the engine and generates a throttle signal representing throttle opening of the engine, a vehicle speed sensing means which measures vehicle speed and generates a vehicle speed signal representing vehicle speed, a shift signal generating means which generates an upshift signal or a downshift signal based upon the balance between the throttle signal and the vehicle speed signal, and an actuator control means which changes over operation of the actuators according to the upshift or downshift signal dispatched from the shift signal generating means so that the transmission is set at the high speed stage when throttle opening and vehicle speed are in a predetermined coordinated high speed range and the transmission is set at the low speed range when throttle opening and vehicle speed are in a predetermined coordinated low speed range. In this case, the coordinated high and low speed ranges are generally so determined that, when vehicle speed is higher than a predetermined relatively high value, the transmission is set at the high speed stage regardless of the value of throttle opening, and, when the vehicle speed is lower than a predetermined relatively low value, the transmission is set at the low speed stage regardless of the value of throttle opening.
In a transmission of this kind there is a problem that, when the depression of the accelerator pedal is rapidly released in order to effect engine braking under such a running condition of the vehicle that the vehicle speed is relatively high while the accelerator pedal is relatively deeply depressed with the transmission thereby still being set at the low speed stage, the balance between throttle opening and vehicle speed is shifted from the coordinated low speed range to the coordinated high speed range due to the abrupt reduction of throttle opening which occurs prior to the corresponding reduction of vehicle speed due to engine braking, and accordingly the transmission is shifted up from the low speed stage to the high speed stage, thereby lessening to a large degree the effect of the engine braking.